1. Field of the Invention
The present invention relates to a luminescent semiconductor device having an active layer composed of a Group II-VI compound semiconductor, a method for making the same, and an optical disk apparatus provided with the same.
2. Description of the Related Art
The requirements for optical recording disks and magnetooptical recording disks in recent years include high recording/regenerating density and high resolution, hence semiconductor devices emitting green or blue color have been demanded.
Prospective semiconductors for semiconductor devices emitting green or blue color include Group II-VI compound semiconductors composed of at least one Group II element selected from zinc (Zn), magnesium (Mg), beryllium (Be), cadmium (Cd), and mercury (Hg), and at least one Group VI element selected from oxygen (O), sulfur (S), selenium (Se), and tellurium (Te). Among them, a ZnMgSSe mixed crystal can be deposited on a GaAs substrate composed of gallium (Ga) and arsenic (As), and it has been known that the crystal is suitable for a guide layer or a clad layer in semiconductor laser devices emitting a blue light laser beam (for example, Electron. Lett., 28, p. 1798 (1992); Electron. Lett., 29, p. 1488 (1993); and Appl. Phys. Lett., 66, p. 656 (1995)).
In general, luminescent semiconductor devices have been fabricated by separately depositing a clad layer composed of a ZnMgSSe mixed crystal, a guide layer composed of a ZnSSe mixed crystal or ZnS, and an active layer composed of a ZnCdSe mixed layer on a GaAs substrate (for example, N. Nakayama et al., Electron. Lett., 16, p. 1488 (1993); and S. Taniguchi et al., Electron. Lett., 32, p. 552 (1996)). Such a luminescent semiconductor device is formed by, Group II particle beam radiation and Group VI particle beam radiation by means of, for example, molecular beam epitaxy (MBE), in which a Group VI element is fed in an amount of 1 to 2 times the amount of a Group II element.
In the above-mentioned conventional luminescent semiconductor devices, it has been found that the wavelength of the luminescence becomes short when energized. FIG. 1 is a graph illustrating the results of the energizing test of a luminescent semiconductor device which consists of a clad layer composed of a ZnMgSSe mixed crystal, a guide layer composed of a ZnSSe mixed crystal, and an active layer composed of a ZnCdSe mixed crystal provided between the clad layer and the guide layer. The device was energized at 40.degree. C. for 30 minutes in a 1 mW auto power control (APC) mode. The emission spectrum after energizing was measured by a low-temperature cathode luminescence (CL) method. FIG. 1 includes an emission spectrum at an energized section wherein a current flows by current constriction, and an emission spectrum at a non-energized section wherein no current flows in the luminescent semiconductor device.
As shown in FIG. 1, the emission spectrum from the active layer shifts by 3 nm toward a shorter wavelength by energizing. The same shift due to energizing is also observed in luminescent semiconductor devices having a ZnSe guide layer, although it is not shown in the drawing. It is thought, as shown in FIG. 2, that the shift toward a shorter wavelength due to energizing is caused by electric diffusion of cadmium in the active layer into the guide layer and of zinc in the guide layer into the active layer. Such diffusion results in a decrease in the cadmium content in the ZnCdSe mixed crystal as the constituent of the active layer.
The diffusion of cadmium from the active layer increases the band gap of the active layer, as shown in FIG. 2, and the increased band gap causes an increase in the operating current flow and a decrease in the characteristic temperature due to carrier overflow. Further, dark line defects (DLD) occur at the position of the diffusion. The occurrence of the DLD suggests the formation of crystal defects, such as dislocation, which function as non-luminescent recombining centers increasing the operating current flow, resulting in deterioration of the device. In addition, nonuniform diffusion of cadmium causes an increase in the half-width of the emission spectrum, resulting in a decreased gain and an increased operating current flow.